For many people in China, the sculptor and installation artist acts as their social conscience. He campaigns for freedom of speech in his home country and publically condemns political oppression. For that reason, the critic is banned from leaving China. For the past three years, authorities have refused to issue him a passport.

"Evidence" is the title of the exhibition, which is taking place in the Martin Gropius Bau in Berlin. With 34 scultpures and installations, Ai Weiwei reflects on the social reality in his country, like with this work, "Wooden Stools." Displayed in the museum's courtyard, the 6,000 stools are organized according to a geometrical pattern.

These wooden stools, which originated during the Ming and Qing dynasties, were collected by Ai Weiwei's assistants in northern China over a number of years. At one time, they could be found in every household and were passed down from generation to generation. Since the Cultural Revolution in the 1960s, stools have been mass-produced using plastic.

In 2011, Ai Weiwei was incarcerated in a prison near Beijing and guarded around the clock for 81 days - without an arrest warrant. The lights were kept on in his cell day and night. He was accused of tax evasion and his passport was revoked. International pressure finally led to his release. For the Berlin exhibition, the artist has recreated his cell in a work titled "81 days."

After his release, the authorities continued their surveillance of Ai Weiwei and installed a dozen cameras in front of his atelier in Beijing. The state is aware of every visitor he receives. He's now turned the situation around, at least artistically, by recreating the surveillance cameras in marble for the Berlin show.

In the past, the Chinese government tried to beneft from Ai Weiwei's popularity. He was encouraged to set up another studio in Shanghai as part of a giant artists' village. In 2011, his criticism of the regime was deemed unacceptable and the completed studio was torn down. The 57-year-old created an artwork with the rubble from the atelier, placing it in a traditional wooden bed.

To protest the demolition of his atelier and annoy the authorities, Ai Weiwei organized a freshwater crab festival. The Chinese word for freshwater crab sounds just like "he xie," the word for harmony. State propaganda promotes harmony as the ideal for Chinese society. Ai Weiwei's porcelain crabs can now be seen in Berlin.

Ai Weiwei welded 150 bicycle frames into an impressive installation. The work is not only a reference to cars taking over the streets in China, but also to a prominent show trial. Several years ago, a young Chinese man was arrested and mistreated for not registering his bicycle. He was later sentenced to death.

About half of the works on show were created especially for the Berlin exhibition, which was set up by Ai Weiwei's personal assistants. The artist is forbidden from leaving China, which is why he sent a video message for the opening: "I hope can come to see the show and can share the moment with my audience." The exhibition runs through July 7, in the Martin Gropius Bau.